Ionic liquids are liquids composed of ions that are fluid around or below 100° C. (Rogers, R. D.; Seddon, K. R. Science 2003, 302, 792-793). Ionic liquids exhibit negligible vapor pressure, which makes them suitable replacements for conventional solvents. Conventional solvents like dichloromethane, acetone, benzene, and methanol are volatile organic compounds which contaminate soil and groundwater, pollute indoor air, and increase the level of greenhouse gases. Increasing regulatory pressure to limit the use of volatile organic compounds has sparked research into designing ionic liquids that could function as environmentally friendly replacement solvents.
By virtue of their high polarity and charge density, ionic liquids have unique solvating properties, and are being used in a variety of applications. In addition to applications include in organic synthesis as a green solvent, ionic liquid are used in novel materials science (liquid crystals, gels, rubbers); electrochemistry, for example, in fuel cells, electrodeposition processes and other electrochemical applications; in applications where water-based chemistry can be problematic (for example, applications involving proton transfer or nucleophilicity); or in applications where certain coordination chemistry could have a damaging effect on the substrates involved.
Some simple physical properties of the ionic liquids that make them interesting as potential solvents for synthesis are the following: (1) They are good solvents for a wide range of both inorganic and organic materials, and unusual combinations of reagents can be brought into the same phase; (2) They are often composed of poorly coordinating ions, so they have the potential to be highly polar yet noncoordinating solvents; (3) They are immiscible with a number of organic solvents and provide a nonaqueous, polar alternative for two-phase systems; and (4) Ionic liquids are nonvolatile, hence they may be used in high-vacuum systems and eliminate many containment problems (Welton, T. Chem. Rev., 1999, 99, 2071-2083).
A broad range of ionic liquids have been investigated in the past. The most commonly studied systems contain ammonium, phosphonium, pyridinium, or imidazolium cations, with varying heteroatom functionality. Common anions that yield useful ILs include hexafluorophosphate, [PF6]−; tetrafluoroborate, [BF4]−; bis(trifyl)imide, [NTf2]−; and chloride, Cl−. Anions can control the solvent's reactivity with water, coordinating ability, and hydrophobicity. Accordingly, ionic liquids are relatively advanced, technological solvents that can be designed to fit particular applications (Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis, VCH-Wiley: Weinheim, 2002; Rogers, R. D.; Seddon, K. R., Eds. Ionic Liquids: Industrial Applications for Green Chemistry, ACS Symposium Series 818, American Chemical Society: Washington, 2002).
Interest in room-temperature ionic liquids (RTILs) has increased enormously during the last decade because, among other applications, they may be used to replace less environmentally friendly (‘green’) solvents. Invariably, these ionic liquids are either organic salts or mixtures consisting of at least one organic component. The most common salts in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium, and N,N′-dialkylimidazolium cations.
A major drawback of many ionic liquids is their air and moisture stability. Many ionic liquids are hygroscopic; if used in open vessels, hydration will almost certainly occur. The degree to which this is a problem will depend on the use to which the ionic liquid is being put and what solutes are being used. For example, the smallest amount of water can deactivate a highly reactive solute used as a catalyst (Chauvin, Y. et al. Angew. Chem., Int. Ed. Engl. 1995, 34, 2698-2700).
Similarly, supercritical carbon dioxide has also been employed as a green solvent although it requires specialized equipment and reaction vessels. Carbon dioxide also shows solubility in many ionic liquids. Although it is known that amidines and alcohols react very rapidly with CO2 to form amidinium carbonate salts, some of which are liquids at room temperature, they are stable only under scrupulously dry conditions (Hori, Y. et al., Chemistry Express 1986, 1, 224-227). Jessop has demonstrated that the fraction of the amidinium carbonate made by bubbling CO2 through a carefully dried 1/1 mixture of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1-hexanol can be returned to its original state by bubbling through N2 or Ar gas (Jessop, P. G. et al., Nature, 2005, 436, 1102). However, the uptake of CO2 in DBU/alcohol systems may not be quantitative at 1 atm pressure (Hori, Y. et al., Chemistry Express 1986, 1, 173-176). Also, bubbling CO2 through a dilute solution of an N′-alkyl-N,N-dimethylacetamidine in water yields an acyclic amidinium bicarbonate, a reversible surfactant (Liu, Y. et al., Science 2006, 313, 958-960).
It is desirable to provide solvent systems whose electrostatic properties can be changed reversibly from relatively low polarity to very high polarity by the addition of different gases. Transparent systems open opportunities for interesting spectroscopic investigations. It is further desirable to develop systems which employ inexpensive precursor amidines and amines, some of which may exhibit liquid crystallinity.